1. Field of the Invention
The present invention relates a charged particle detection device used in an evaluation apparatus for a semiconductor device or the like and a charged particle radiation apparatus using the charged particle detection device.
2. Description of the Related Art
In recent years, with an increase in degree of the integration density of the semiconductor device, the layout of the semiconductor device has been more finely micropatterned. For example, in a scanning electron microscope for pattern size measurement (size SEM) or the like evaluating the semiconductor, a method of evaluating a fine pattern at high accuracy must be strongly demanded. For this reason, in addition to a conventional method of detecting a secondary electron signal from a sample by a scintillator to evaluate the sample, a method in which a reflected electron signal from the sample is utilized has been developed.
A reflected electron signal is rarely affected by charge-up or contamination of a sample, and the energy dependence of the contrast of a reflected electron image is low. For this reason, evaluation can be stably performed at high accuracy using the reflected electron signal. Since reflected electrons generally have a signal amount smaller than that of secondary electrons, the performance of a size SEM or the like is in proportion to the number of reflected electrons received by a detector.
For example, FIG. 1 shows a method of using a conventional reflected electron detector. A detector 101 processed to have a thin-plate shape is arranged between an objective lens 103 of a scanning electron microscope 102 and a sample 104. An electron beam 105 from the scanning electron microscope 102 is radiated onto the sample 104 through a hole 106 formed in the central portion of the detector 101, and reflected electrons generated by the sample 104 are received by the detection surface opposing the sample 104, thereby improving the capturing yield of the reflected electrons. As a conventional reflected electron detector used in the above arrangement, a photodiode or a multi-channel plate (MCP) is used.
In an evaluation apparatus such as a scanning electron microscope (SEM) using an electron beam, it is known that a resolving power is increased by decreasing the working distance between an objective lens and a sample. For this reason, this working distance will have to be 1 mm or less in the future. Of limiting conditions of the apparatus, the thickness of a reflected electron detector is important. In this point, a photodiode which can be formed in a thin-film silicon wafer is advantageous more than an MCP whose thickness cannot be decreased without degrading the sensitivity of the detector. In addition, the photodiode is better than the MCP in stability or reliability because the photodiode does not require a high voltage and is rarely contaminated when the photodiode is exposed to the air.
The photodiode is constituted such that a p.sup.+ -type layer 112 and an n.sup.+ -type layer 113 are formed on one surface of an n-type semiconductor substrate 111 and the other surface thereof, respectively. The photodiode is a light-receiving element for generating a current or a voltage by radiating light onto a p-n junction portion formed by the n-type semiconductor substrate 111 and the p.sup.+ -type layer 112. This photodiode generates electron-hole pairs by radiating not only light but also high energy electrons onto the p-n junction portion. Of electron-hole pairs generated near a depletion layer 114 of the p-n junction portion or diffused to the depletion layer 114, the electrons and holes are moved to the n-type layer and the p-type layer by an internal electric field, respectively. The p-type side and the n-type side operate as positive and negative outputs, and the electrons and holes can be extracted therefrom.
However, when electrons are detected by a photodiode, unlike when light is detected by the photodiode, both reflection of incident electrons onto the surface of the photodiode and attenuation caused when the incident electrons are transmitted through the surface layer of the photodiode cannot be neglected. More specifically, since the surface layer must have a thickness of about 0.5 .mu.m, as shown in FIG. 3, electrons having an energy of about 6 keV or less cannot be detected. On the other hand, in an evaluation apparatus using an electron beam, the acceleration energy of an electron beam conventionally used at 20 to 30 keV must be suppressed to 1 to 2 keV or less to prevent radiation damage and charge-up of a substrate. In this case, since a reflected electron energy decreases in proportion to an incident electron energy, the reflected electrons cannot be detected by a conventional photodiode.